It is not only the GPS receivers but the internal programming of the unit coupled with the satelites and their "talk" capabilities. Fewer satelites and or fewer receiver components will affect position determination, update and display.
With HB products I suspect that things like sonar and mapping can take away from the processor in its timings to service modules such as GPS.
We have to rememer that these things are computers and operate just like them.
Slow processor speeds, slow bus speeds or anything that slows the process (instruction execution) will affect the total operation.
I am guessing that sonar may take precedence over GPS (to a degree)
I have not tried to use just the GPS and no sonar..
Chuck